1. Field of the Invention
The present invention relates to a resonator for suppressing the intake noise of an intake system for a vehicle.
2. Related Art
A side branch resonator or a Helmholtz resonator has been used in the related art in order to suppress intake noise of an intake system. Such a related art resonator has a disadvantage that a larger installation space for a resonator is required in case the sound pressure of a lower frequency component with lower frequency of intake noise is to be suppressed.
For a side branch resonator, the natural frequency of sound that can be silenced by resonance depends on the length of the side branch. Meanwhile, the wavelength becomes longer as the signal component becomes lower. In order to suppress a low frequency component by using a side branch resonator, the side branch length must be increased. This increases the installation space for the resonator.
For a Helmholtz resonator, the natural frequency of sound that can be silenced by resonance is represented by the following expression:
                    f        =                              c                          2              ⁢              π                                ⁢                                    S                              I                ·                V                                                                        (                  Expression          ⁢                                          ⁢          1                )            
In the above expression, f represents a natural frequency (resonance frequency), c a sound velocity, l the length of a communication pipe, V the volume of a cavity chamber, and S the cross-sectional area of the communication pipe. To suppress a low frequency component, it is necessary to reduce the natural frequency f. To reduce the natural frequency f, it is necessary to increase l or V with respect to S. In this case also, the installation space for the resonator is increased.
A resonator having a small installation space is described in JP-UM-A-2-080710. The resonator comprises an elastic film and a cup member. The cup member is attached to a surge tank with the cup opening turned down. Between the cup opening and the surge tank is interposed an elastic film. The elastic film separates the cup interior from the surge tank interior.
The natural frequency of the elastic film is set to be equal to the resonance frequency of columnar resonance in the surge tank. The resonator described in JP-UM-A-2-080710 is capable of suppressing columnar pulsation in the surge tank by way of the film vibration effect of the elastic film.
A problem with the resonator described in JP-UM-A-2-080710 is that it is difficult to maintain a desired sound pressure suppression effect for a substantial period of time. In other words, the natural frequency of an elastic film must be constantly maintained to be equal to the frequency of the resonance frequency of columnar resonance. The natural frequency of the elastic film depends on the tension of the elastic film. The tension of an elastic film gradually decreases with time from when the elastic film is installed. Thus, it is difficult for the resonator described in JP-UM-A-2-080710 to maintain a desired sound pressure suppression effect for a substantial period of time.